Strong climate and tectonic control on plagioclase weathering in granitic terrain

Investigations to understand linkages among climate, erosion and weathering are central to quantifying landscape evolution. We approach these linkages through synthesis of regolith data for granitic terrain compiled with respect to climate, geochemistry, and denudation rates for low sloping upland profiles. Focusing on Na as a proxy for plagioclase weathering, we quantified regolith Na depletion, Na mass loss, and the relative partitioning of denudation to physical and chemical contributions. The depth and magnitude of regolith Na depletion increased continuously with increasing water availability, except for locations with mean annual temperature < 5 °C that exhibited little Na depletion, and locations with physical erosion rates < 20 g m− 2 yr− 1 that exhibited deep and complete regolith Na depletion. Surface Na depletion also tended to decrease with increasing physical erosion. Depth-integrated Na mass loss and regolith depth were both three orders of magnitude greater in the fully depleted, low erosion rate sites relative to other locations. These locations exhibited strong erosion-limitation of Na chemical weathering rates based on correlation of Na chemical weathering rate to total Na denudation. Sodium weathering rates in cool locations with positive annual water balance were strongly correlated to total Na denudation and precipitation, and exhibited an average apparent activation energy (Ea) of 69 kJ mol− 1 Na. The remaining water-limited locations exhibited kinetic limitation of Na weathering rates with an Ea of 136 kJ mol− 1 Na, roughly equivalent to the sum of laboratory measures of Ea and dissolution reaction enthalpy for albite. Water availability is suggested as the dominant factor limiting rate kinetics in the water-limited systems. Together, these data demonstrate marked transitions and nonlinearity in how climate and tectonics correlate to plagioclase chemical weathering and Na mass loss.

Research Highlights
► The depth and magnitude of regolith Na depletion increased with water availability.
► Cold and low erosion rate location Na depletion unrelated to water availability.
► Grade from kinetic to erosion limited weathering with increased water availability.
► Variable sodium activation energy for erosion- and kinetic-limited locations.